The name "integrated optical systems" has become generic to monolithic thin-film structures designed to process light signals and obtained by techniques of depositing, diffusion and etching, using masking operations and similar to those employed in the manufacture of integrated electronic circuits. It is possible in particular, using these techniques, to build linear structures characterized by a refractive index which is higher than that of the surrounding medium, and forming wave guides along which light propagates in accordance with a series of total reflections or progressive refractions.
In the prior art, it is known to combine two such waveguides by arranging them parallel to one another over part of their length in order to form directional couplers; through the medium of the evanescent wave phenomenon, the energy carried in the first waveguide is transferred progressively to the second waveguide and a maximum energy transfer is observed at the end of a certain length known as the coupling length, which depends upon the geometric and optical parameters of the structure and in particular upon the value of the refractive indices of the materials constituting the two waveguides, as well as that of the medium separating them; subsequently, the energy transfers progressively from the second waveguide to the first and so on. It is also known, by utilizing an electro-optical material, for one of the materials constituting the waveguide or the material which separates them, to vary the refractive index under the effect of an electric field, and thus by changing the coupling length, to electrically control the energy proportion transferred from one waveguide to the other; it is also possible, using this same principle, to form a light modulator by arranging parallel to the waveguide which carries the light-wave, a section of waveguide to which a greater or lesser proportion of said energy is transferred.
It has been recognized that the solution which requires the minimum control voltage for a given coupler, is that which utilizes two identical rectilinear waveguides, one of them being imparted a given variation in refractive index and the other a variation of the same amplitude but opposite sign.
To achieve this condition, it has been proposed to arrange parallel to the waveguides, in the coupling zone, three electrodes, one between the two waveguides and the two others at either side thereof; it is thus possible to subject the two waveguides of the coupler to electric fields of the same values but opposite directions. However, the need to reduce to some few wave lengths the interval between the waveguides and the coupling zone, imposes a very narrow width and consequently higher resistance, on the central electrode; since the spread capacitance of the system constituted by the three electrodes is not negligible, the long time constant of the circuit limits the latter to switching or modulating frequencies which are relatively low. Moreover, the presence of the central electrode, however narrow it may be, leads to an increase in the spacing between the waveguides and this, by reducing the coupling efficiency, increases the length of the coupler.
It has also been proposed, again in order to achieve opposite variations in refractive index in the two waveguides, that the earlier mentioned device with three electrodes should be used in order, at the time of manufacture of the coupler, to prepolarize in two directions, perpendicular to the wave guides, the material of which these later are made; with this objective in mind, the device as a whole is raised to a temperature in excess of the Curie point of the ferroelectric material constituting the waveguides; then, a voltage is applied between the central electrodes and the lateral electrodes while the assembly is slowly cooled. The central electrode is then discarded and the control voltage applied to the two sole remaining lateral electrodes, creates an electric field which, passing through the two waveguides in the same direction, is co-directional in one of them with the polarization vector and is oppositely directed thereto in the other; this field thus brings about oppositely directed variations in refractive index in the two waveguides of the coupler. However, as in the device described earlier, the temporary presence of a central electrode dictates a certain spacing between the two waveguides. Moreover, in order to manufacture this kind of coupler, it is necessary to carry out a high-temperature treatment on the overall integrated optical circuit, and this complicates the design and may not be compatible with the presence of other elements in the circuit.